This invention relates to ceramic heaters for use in electrical smoking articles. More particularly, this invention relates to heaters having multiple-layer ceramic substrates capable of supporting a plurality of resistive heating elements.
A type of electrical smoking article is described in commonly-assigned U.S. Pat. No. 5,060,671, and commonly-assigned U.S. patent application Ser. No. 07/943,504, filed on Sep. 11, 1992, which are hereby incorporated by reference. In this type of smoking article, a tobacco flavor medium is heated as a result of a transfer of thermal energy from a heating element. As the tobacco flavor medium is heated, a smoker at the mouth or downstream end of the smoking article draws air in and around the heated tobacco flavor medium by inhaling, and thereby receives a tobacco-flavored aerosol or vapor.
In order to produce a tobacco-flavored vapor or aerosol, an electrical smoking article must be capable of elevating the temperature of a tobacco flavor medium to at least 400.degree. C. preferably to a temperature in the range of from about 400.degree. C. to about 650.degree. C. The smoking article should allow the smoker to draw naturally, and should provide a tobacco-flavored aerosol or vapor with little delay after draw. To provide rapid delivery of a tobacco-flavored aerosol or vapor, an electrically powered resistive heating element disposed within the smoking article should be capable of reaching the aerosol-generating temperature within 2 seconds, preferably in about 1 second.
Batteries that are suitable for use in smoking articles have electrical characteristics that require the resistance of the heating elements to be in a relatively narrow range, typically between about 1 .OMEGA. and about 4 .OMEGA.. Since the smoking article should preferably be similar in size to a conventional cigarette, it would be advantageous to provide a heater that is relatively compact. However, it is also important for the heater to have sufficient mechanical strength to enable it to withstand frequent handling by a smoker. It has proven difficult to provide a heater for use in a smoking article having the required combination of resistance, size and mechanical strength.
Many ceramic materials have exceptional thermal properties, and accordingly, they have been used as insulating substrates for printed or adhered solid resistive heaters. However, it has been found that the mass of the substrate is a limiting factor in the effective transfer of heat from the resistive heating elements to a tobacco flavor medium in a smoking article. When the mass of the substrate is too great, the substrate will absorb a large amount of the heat generated by the resistive heating elements. If the thickness of the ceramic substrate is reduced, mechanical strength is sacrificed.
In view of the foregoing, it would be desirable to provide a heater, and a process for fabricating a heater, having a thermally stable, multiple-layer ceramic-substrate.
It would also be desirable to provide a heater, and a process for fabricating a heater, which minimizes heat transfer to the environment, without sacrificing a substantial amount of mechanical strength.
It would further be desirable to provide a -process for fabricating a heater that uses known green ceramic tape technology.
It would still further be desirable to provide a heater, and a process for fabricating a heater, which includes convenient mechanical and electrical interfaces.
It would even further be desirable to provide a heater, and a process for fabricating a heater, which is suitable for heating a tobacco flavor medium to a temperature in the range of from about 400.degree. C. to about 650.degree. C. in about 1 second.